This invention relates to electrical components and assemblies and to their manufacture.
The invention is more particularly concerned with leaded electrical components for use in surface mounted assemblies.
Surface mounted components are rapidly taking the place of dual-in-line components for many applications because of the more compact arrangements and the ease of automated mounting that is possible with such components.
Surface mounted components can be either leadless or leaded. The leadless components have contact pads formed around their surface which are directly joined to contact pads on a board or substrate. These leadless components give the most compact configuration, but suffer from the disadvantage that there is little compliance in the mounting of the component with the substrate. Vibration and differential thermal expansion of the component relative to the board can cause stress in the solder joint, with possible damage or increase in joint resistance. Thermal expansion is especially a problem with high-power components.
Leaded components reduce this problem to a certain extent. These leaded components generally have a flat lead of rectangular section which is bent down towards the contact pad. In one form, the leads are bent to form a loop of `J` shape where they abut the contact pads. Alternatively, the leads may be bent to form a flat foot parallel to the contact pad. The length of the lead allows flexing in the plane including the bend in the lead, enabling relative movement to occur between the component and the substrate in a vertical direction and in a horizontal direction at right angles to the edge of the component from which the lead extends. The width of the lead, however, means that it is relatively stiff across its width so that there is very little compliance in the horizontal direction laterally of the lead, that is, parallel to the edge of the component.
A further disadvantage of the previous arrangements arises because the lead is bent to form a loop or flat foot, so that a side of the lead contacts the contact pad on the substrate. This gives a large area of contact, but can cause spreading of the solder paste used to make the joint, beyond the contact pad, when the lead is pushed down onto the contact pad. This can result in the bridging of adjacent contact pads. It will also be seen that, because the width of the contact pads must be at least equal to the width of the leads, the separation between adjacent contact pads is dictated by the spacing between the leads on the component and can be very narrow. This difficulty is increased because the width of leaded components varies from manufacturer to manufacturer and because the width of the contact pad must, therefore, be large enough to accommodate the widest lead.